Sunday, November 27, 2005

--------------------------------------------------------------------------------Chemistry is made intricate and beautiful by the fact that each element reacts in a different way.Even the ancient chemists saw evidence of quantum structure in the properties of the elements. They didn't see these properties directly, nor did they have the atomic numbers right until Moseley came along, but we know what the sizes of the atoms look like as a function of Z

What are the atomic numbers of the relatively smallest sized atoms? What do these atomic numbers (number of electrons) correspond to?

We know an even more direct measure of the atoms energy levels than that of the size of the atom. (How are size and energy related?) The Ionization Energy (or Ionization Potential) can be thought of as the negative of the energy of the highest occupied atomic orbital. The IP should increase as Z2 within a shell, if electron-electron repulsion were completely ignored.

The IP is really the Heat (Enthalpy change) of the following reaction

X(g) = X(g)+ + e- DH = IP

The Ionization Potential roughly parallels the non-metallic character of the elements, in that the easier it is to remove an electron from an element, the more metallic it is.

The first ionization potential is the energy required to remove one electron from the atom, but this process can be repeated until all the electrons are gone.

X(g)+(y-1) = X(g)+y + e- DH = IPyThe IP increases with the degree of ionization simply because of the extra energy it takes to remove an electron from an increasingly large positive charge. But, the structure of the atomic energy levels can be clearly seen upon multiple ionization of a single element or examining the periodic properties of ions of the same charge.

The Electron Affinity is defined as DH for the process

X(g) + e- = X(g)- DH = EA

Here are some E.A.'s for the main group elements. Note, some elements do not support stable negative ions, i.e. have positive EA's. (All elements have positive ions)

--------------------------------------------------------------------------------Pauling introduced a concept that combined the IP and the EA of an element into something he called the electronegativity, which reflects a measure of how greedy for electrons an atom will be in chemical interactions with other elements.

The small 'tight' atoms have large electronegativity and the big 'squishy' atoms have low electronegativity as is shown with the dots sizes on the bargraph below:

We will use the electronegativity in our discussion of how polar a chemical bond is between unlike elements because it is a good measure of how unequally bonding electrons will be shared between atoms.

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